Mode-locked phase-stabilized laser device

ABSTRACT

In a mode-locked laser device the phase shift of the output pulse and the mode locking signal are compared. The latter signal is controlled in response to this comparison to thereby maintain the laser output pulse in phase with the mode locking signal.

United States Patent Ueno et a]. [4 1 Mar. 21, 1972 [54] MODE-LOCKEDPHASE-STABILIZED LASER DEVICE [56] References Clted [72] lnventors:Yoshlto Ueno; Youlchl Matsumoto, both of UNITED STATES PATENTS T9195Japan 3,521,069 7/1970 DeMaria et al. ..331/94.s [73] Assig'nee: NipponElectric Co., Ltd., Tokyo, Japan I Primary Examiner-William L. Sikes[22] July 1970 Attomey-Sandoe, l-lopgood and Calimafde 211 Appl. No.:57,641

[57] ABSTRACT [30] For i n A li ti P i -fl D m in anode-locked laserdevice the phase shift of the output I pulse and the mode locking signalare compared. The latter y 24, 1969 Japan "4458624 signal is controlledin response to this comparison to thereby maintain the laser outputpulse in phase with the mode locking 52 us. Cl ..331/94.s signaL [51]Int. Cl [58] Field of Search ..33 1 [94.5; 350/ -l 60 4 Claims, 4Drawing Figures I L 1 I I5 a o i-ZEE: 1-:(5 55:12 I

Amplifier Amplifier Variable A Elissa FIT Phose N n Shifter 'DetectorDeloy PAIENTEHHARZI i972 3,651,424

SHEET 1 or 2 Am lif'e l (Prior Ari) p I r O 9 FlG.l L

- 'r L I l l5 s f O :r:

-0 0+ Amplifier Amplifier d A Variable A P I'T e 7 Phase Shifter Shifter7 Deteclor Delay F G. 2 uvvavroes YOSHITO UENO YOUICHI MATSUMOTO y jw,@KM

ATTORNEYS MODE-LOCKED PHASE-STABILIZED LASER DEVICE This inventionrelates generally to mode-locked lasers and, more specifically, to alaser having means for stabilizing the phase of light pulses generatedby the laser.

In a gas laser, a mixture of helium-neon gas is subjected to pumping toreach a negative temperature state, whereby the population inversion ofenergy level is achieved. The gas mixture is disposed between tworeflective mirrors which form a resonator. One of the. reflectivemirrors has a transparency of the order of l percent and serves as theoutput mirror. The laser output beam includes multiple transverse modesfor each longitudinal mode, as well as multiple longitudinal modes foreach transverse mode. This results in the interference between thevarious mode components due to the nonlinear property of the lasermedium, and the production of undesirable spurious frequency components.

In an attempt to avoid this interference, the radius of curvature ofeach reflective mirror is determined so that laser oscillation iseffected only in a selected transverse mode, and the diameter of thelaser beam is adjusted so as to suppress the un desirable transversemodes. However, this does not fix the oscillation frequency intervalpeculiar to a selected longitudinal mode. As a result, undesirableoscillation frequency components are left unremoved in the laser outputbeam.

To overcome this difficulty, various proposals have been made, such as amode-locked laser having an internal light modulator. In this laser, amode locking signal is applied to the modulator. The frequency of thatsignal is set at the frequency interval between the spectra peculiar toa longitudinal mode to be selected. This procedure is generallydesignated forced mode locking. As a result of the forced mode locking,the laser generates a train of light pulses at a fixed repetitionfrequency equal to the mode locking signal frequency. It has, however,been found that the phase relationship between the modelocked outputlight pulse and the mode locking signal changes over a period of time.This phase deviation is attributable to various factors, such as adecrease in .the oscillation intensity due to a decrease in gaspressure, an increase in the internal loss due to a misalignment of theoptical axes of the reflecting mirrors, and a change in the refractiveindex of the internal modulator. Such phase deviation causes thecollapse in the synchronized state, to thereby produce unwanted noise,particularly when the laser is used in a ultrahigh speed PCM lightcommunication system.

It is therefore an object of the invention to provide a modelocked laserdevice in which the laser output pulse is always maintained in phasewith the mode locking signal.

It is another object of the invention to provide a modelocked laserdevice in which undesired frequency components in the laser output beamare eliminated.

It is a further object of the invention to provide a modelocked laserdevice that is admirably well suited for use in an ultra high speed PCMtransmission system.

In the laser device of this invention, the output light pulse train isphase compared with the mode locking signal. The phase of the modelocking signal is controlled in response to the result of the phasecomparison, and is then applied to the internal modulator so that theoutput light pulse may be brought in phase with the mode locking signal.As a result, the output light pulse is always maintained in phase withthe mode locking signal.

To the accomplishment of the above and to such further objects as mayhereinafter appear, the present invention relates to a mode-lockedphase-stabilized laser device substantially as defined in the appendedclaims, and as described in the following specification taken togetherwith the accompanying drawings in which:

FIG. 1 is a schematic diagram of a conventional laser device utilizingforced mode locking;

FIG. 2 is a schematic diagram of a laser device according to anembodiment of this invention;

FIG. 3 illustrates the frequency spectrum of the output of the laserdevice of FIG. 1; and

FIG. 4 illustrates the characteristic curves for describing theoperation of the embodiment of FIG. 2.

In the prior art embodiment shown in FIG. 1, a laser generallydesignated 1 includes a laser tube 2 containing, for example, ahelium-neon gas mixture. Windows 3 and 4 are formed at Brewsters angleat both ends of laser tube 2, and reflecting mirrors 5 and 6 aredisposed at both ends of laser tube 2 and respectively face windows 3and 4. An internal light modulator 7, made of an electro-optical elementsuch as a KDP crystal, is disposed between reflecting mirror 6 and lasertube 2. The laser when suitably actuated by suitable means (not shown)generates a light beam which is indicated in FIG. 1 by the dotted line8.

A mode locking signal is applied from a terminal 9 to a modulator 7,after being amplified at a power amplifier 10. The structure and mannerof operation of the electro-optical effect modulator are well known tothose having ordinary skill in this technical field and will thereforenot be further described herein. Stated briefly, modulator 7 serves as aswitching means for the light beam, based on the rotary and linearpolarization such as described in US. Pat. No. 3,467,915.

The principles of mode locking will now be briefly described. Assumingthat the spacing between mirrors 5 and 6 is L and that the laser isoperating in a plurality of longitudinal modes coexistent in a singletransverse mode, the laser output will have the spectra as shown in FIG.3, in which the frequency spacing between the frequency components isdefined by c/2L (wherein c is the speed of light). This shows that asingle light pulse is reciprocated between the two reflecting mirrors atthe light velocity c, andthe output is produced every time the singlelight pulse reaches the reflecting mirror on the output side.

When a mode locking signal having a frequency of c/2L is applied to themodulator to modulate the loss of the resonator, those frequencycomponents surrounding the desirable mode components are forced to jointhe mode components. Thus, the repetition frequency of the output lightpulse is made equal to the mode locking signal. This is generallyreferred to as the pull-in effect. (For further details see: U. Uchida,A. Ueki Self Locking of Gas Lasers, IEEE J. QE-3, No. l, p. 17, Jan.1967; W. R. Bennett, Jr. Hole Burning Effects in a l-Ie-Ne OpticalMaser, Phys. Rev., Vol 126, p. 580, Apr. 1962; W. R. Bennett, Jr.Gaseous Optical Masers, Appl. Optics, supplement 1, p. 24, 1962).

In the prior art laser device of FIG. 1, however, it has been found thata phase deviation is developed between the mode locking signal and theoutput light.

This can be described by referring to FIG. 4 in which the frequency f ofthe mode locking signal is plotted along fl the abscissa, and the phasedeviation of the output pulse with respect to the mode locking signal isplotted along the ordinate. Curves l2, l3 and 14 illustrate the modelocking frequency vs. phase deviation characteristics respectively for alaser output of 300 mw., 500 mw. and 700 mw. Needless to say, the rangeof the mode locking signal frequency in which self-mode-locking isunaffected is rather limited. As will be seen from these curves, thephase of the light pulse is advanced for high mode locking signalfrequencies. In contrast, when the frequency f is decreased, the phase 0lags. Likewise, for a fixed mode locking signal frequency fl, the phaseof the output light beam tends to advance with the decrease of theoscillation intensity. This is shown by the fact that the phasedeviation 0! for point a on curve 14 is smaller than the deviation 02for point b on curve 13. (For a theoretical analysis of this phenomenon,see T. Uchida: Dynamic Behavior of Gas Lasers, IEEEL, QE-3, No. lp.7,Jan. 1967). This shows that a very limited variation of oscillationintensity due to internal loss appreciably afiects the phase of thelight pulse output. Therefore, if the reflecting mirror is slightlymisaligned with respect to the optical axis of the laser due, forexample, to ambient temperature change or degradation in the reflectionfactor of the reflecting mirrors due to dust fallout, the phase of theoutput light pulse is greatly affected.

This invention is therefore based on the phase control of the modelocking signal carried out so that it follows the change in the phase ofthe output light pulse.

Referring to the embodiment of the invention depicted schematically inFIG. 2, a variable phase shifter l 1, consisting of a variablecapacitance diode or the like, is interposed between mode locking signalinput terminal 9 and amplifier 10. The oscillation output of the laserthrough mirror is envelope-detected by a photodiode placed adjacentmirror 5 into an electrical signal. The transparency of mirror 5 issufficiently small so as not to affect the laser oscillation and may be,for example, as low as 0.1 percent. The detected output of photodiode 15is amplified by a VHF band amplifier 16 and is then applied to a fixedphase shifter 17 which provides a certain definite phase shift. Phaseshifter 17 may consist of lumped constant circuit elements such asinductive and capacitive elements. The phase-shifted detected output issup plied to a phase detector 18, to which a portion of the mode lockingsignal is supplied from terminal 9 through a delay means 19.

Phase detector 18 detects the phase difference 0 between the incomingmode locking signal and the output light pulse. The extremely lowfrequency output voltage of detector 18 is proportional to cos 0. Inthis case, the phase shift to be brought about by fixed phase shifter 17is determined so that the phase difference between the two signalsapplied to phase detector l8 may be 90. The phase-comparison output ofdetector 18 is applied to the variable capacitance diode of variablephase shifter 11. The phase-controlled mode locking signal issufficiently amplified by amplifier l0, and is applied to modulator 7.As will be seen, from the embodiment of FIG. 2 the phase of the lockingsignal is controlled so as to minimize the phase deviation of the outputlight pulse. Thus, the phase of the output light pulse is stabilized orforced to closely follow that of the mode locking signal. Delay line 19is employed so that time relationship between the two outputs of phaseshifter 17 and terminal 9 is regulated.

For the purpose of stabilizing the phase of the output light pulse itmay be conceived that the loss of oscillation is changed according tothe variation of the detected phase. In other words, it may beconsidered that the output of the phase detector 18 is superimposed onthe bias voltage of the internal modulator 7 in the prior an embodimentof FIG. 1. In this case, however, the output light pulse amplitude isalso affected and, in some cases, the oscillation may become unstable.However, in the arrangement of this invention, as shown in FIG. 2, thelaseroscillation itself is not affected at all, because only a portionof the oscillation is detected.

As is apparent from the foregoing description, this invention makes itpossible to keep the laser output light pulse exactly in phase with themode locking signal. This insures the maintenance of synchronism whenthe device is used for ultrahigh speed PCM transmission system.

Thus while only a single embodiment of the present invention has beenherein specifically described it will be apparent that modifications maybe therein without departing from the spirit and the scope of theinvention.

We claim:

1. In a phase stabilized laser device for forced mode locking operationof a laser having a laser active material, a pair of reflecting mirrorseach facing said laser active material, an internal optical modulatorinterposed between said laser active material and one of said pair ofreflecting mirrors, and a source of a mode locking signal to be suppliedto said optical modulator; the improvement which comprises: means fordetecting the output light pulse of said laser, means coupled to saidlight detecting means for detecting the phase difference between thedetected light pulse and said mode locking signal, and means disposedbetween said mode locking signal source and said optical modulator forvariably controlling the phase of said mode locking signal in responseto the output of said phase difference detecting means.

2. The improvement of claim 1, further comprising fixed ghase differenceshifting means 0 eratively interposed etween said light detecting meansan said phase detecting means.

3. The improvement of claim 1, further comprising means coupled to saidsource for regulating the time relationship between the output of saidphase shifting means and said source.

4. The improvement of claim 3, in which said time regulating meanscomprises delay means coupled between said source and said phasedetecting means.

o UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3651,424 D t d March 21, 1972 Inventor(s) Yoshito Ueno and YouichiMatsumoto It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 4, Claim 2, line 32, "difference" should not have been printed;

line 33, "phase detecting" should have been phase difference detectingSigned and sealed this lst day of August 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JRQ ROBERT GUTTSCHALK Attesting Officer Commissionerof Patents

1. In a phase stabilized laser device for forced mode locking operationof a laser having a laser active material, a pair of reflecting mirrorseach facing said laser active material, an internal optical modulatorinterposed between said laser active material and one of said pair ofreflecting mirrors, and a source of a mode locking signal to be suppliedto said optical modulator; the improvement which comprises: means fordetecting the output light pulse of said laser, means coupled to saidlight detecting means for detecting the phase difference between thedetected light pulse and said mode locking signal, and means disposedbetween said mode locking signal source and said optical modulator forvariably controlling the phase of said mode locking signal in responseto the output of said phase difference detecting means.
 2. Theimprovement of claim 1, further comprising fixed phase differenceshifting means operatively interposed between said light detecting meansand said phase detecting means.
 3. The improvement of claim 1, furthercomprising means coupled to said source for regulating the timerelationship between the output of said phase shifting means and saidsource.
 4. The improvement of claim 3, in which said time regulatingmeans comprises delay means coupled between said source and said phasedetecting means.